This project is focused on the identification of physiologically critical functions and mechanisms of action of NF-kB transcription factors and their regulators in health and disease. NF-kB is a family of related dimeric transcription factors that serve as primary intracellular mediators during innate and adaptive immune responses. In addition, and importantly, aberrant regulation of NF-kB plays a major role in inflammatory and autoimmune diseases as well as in numerous tumors. It is thus imperative to understand the functions and mechanisms of action of individual NF-kB factors and their regulators, as this will be required to devise appropriate strategies for therapeutic interventions aimed at curtailing aberrantly regulated NF-kB in a precisely targeted manner. To identify physiologic roles and mechanisms we make use of mouse models engineered to lack components of the NF-kB transcription factor family or their regulators, as well as models in which the NF-kB factors can be selectively activated. Our work is focused on alternatively and classically activated NF-kB factors, and especially on Bcl-3. The alternative NF-kB activation pathway is normally initiated by a subset of TNF receptors. Bcl-3 is an atypical IkB family member that functions as nuclear regulator of NF-kB activity. In the recent past we discovered a critical role for Bcl-3 in the ability of dendritic cells to properly prime T cells in culture to proliferate in response antigen. Priming of T cells by dendritic cells is crucial to initiate adaptive immune responses. In FY 2015 we have now demonstrated that loss of Bcl-3 in dendritic cells in mice leads to fatal susceptibility of these mice to infection with Toxoplasma gondii, an intracellular pathogen that is normally well-controlled by the host. This pathogen constitutes a serious health risk to immunocompromised patients. In FY 2015 we also demonstrated that Bcl-3 has critical functions in keratinocytes and other stromal cells in skin to help delimit hypersensitivity reactions. In FY 2015 we furthermore discovered that Bcl-3 plays an important role in controlling the plasticity of effector T cells and importantly, that Bcl-3 is crucial to establish and maintain the pathogenicity of auto-reactive T cells. Bcl-3 was required in T cells to maintain a T helper 1-like pathogenic effector state of these cells in the context of experimental autoimmune encephalomyelitis, a model for Multiple Sclerosis, and in the context of T cell transfer-induced colitis, a model for Inflammatory Bowel Disease.